1. Field of the Invention
The present invention relates to an image pickup camera for use with an X-ray fluoroscopic apparatus and, more particularly, to offset compensation thereof.
2. Description of the Related Art
There is an X-ray fluoroscopic apparatus which converts an X-ray image of an object under examination irradiated with X-rays emitted from an X-ray tube into an optical image by use of an image intensifier, picks up the optical image with a television camera, and visually displays the fluoroscopic image output from the television camera on a display. In recent years, in television cameras, solid-state pickup devices using CCDs (charge coupled devices) or MOSs (metal oxide semiconductors) have been replacing conventional image pickup tubes.
FIG. 1 illustrates a schematic arrangement of a CCD solid-state pickup device of an interline system as an example of such solid-state pickup devices. A large number of light receiving elements 90 formed of photodiodes are arrayed two-dimensionally. Part of the light receiving elements, in this example, the leftmost column 90a to which hatching is made (plural columns may be used), is shielded from light to determine the level of an offset signal and is thus referred to as an optical black portion. Vertical transferring CCDs 92 are provided to the left of respective columns of light receiving elements including the optical black column 90a and outputs of all the vertical transferring CCDs 92 are coupled to a horizontal transferring CCD 94. Vertical transferring CCDs 92 and horizontal transferring CCD 94 are also shielded from light. Outputs of all the light receiving elements 90 are transferred to corresponding vertical of repeated vertical transfer and horizontal transfer, an output image signal from horizontal transferring CCD 94 is applied to a succeeding display and so on via a buffer 96.
Light receiving element 90 may produce some amount of signal (offset signal) while it receives no light. For obtaining a true image signal, therefore, it is required to subtract the offset signal from the output of buffer 96. Subtracting the offset signal from the camera output is called the offset compensation. For the offset compensation, as shown in FIG. 2, the level Vb of an offset signal from optical black portion 90a which appears at the beginning of each horizontal scanning line signal is sampled and held to subtract the hold value from the succeeding horizontal scanning line signal.
Heretofore, as described above, an offset signal is obtained for each horizontal scanning line and the offset signal is subtracted from a signal of each of pixels on the horizontal scanning line for the offset compensation. However, the offset component varies from pixel to pixel because of variations in manufacturing of the light receiving elements. With the conventional compensating method, therefore, there is a problem of impossibility of accurate compensation for each pixel.
The above description was given of a solid state pickup device. The same problem may arise with image pickup tubes because of variations in coating of a photoelectric film on a photoelectric surface. The same may be said of any applications of the image pickup camera, other than medical apparatus such as a fluoroscopic apparatus.